Remove abuse report notes and user mentions code

What does this MR do and why?

Removes the dead abuse-report notes and user mentions code. No database changes — this is step 1 of 3 re-introducing the removal that was reverted after INC-13070.

Why this code is dead

Notes on abuse reports were attempted twice and abandoned both times:

1. In the shared notes table, behind the abuse_report_notes feature flag, which was never enabled. Those rows were deleted in 17.2 because notes could not be sharded by namespace while they existed: !155327 (merged)
2. In a dedicated abuse_report_notes table, dropped in 18.7 under a breaking-change exception — "never released to any customers or instances, so they represent dead code": !209042 (merged) and gitlab-com/Product#14362 (closed)

The sibling tables went with it — abuse_report_assignees in 18.5, abuse_report_labels in 18.6 — but abuse_report_user_mentions and the model wiring that fed it were missed.

Nothing can reach that wiring today. An abuse-report note cannot be created at all: Note requires a namespace for non-project noteables (app/models/note.rb), and does_not_exceed_notes_limit? calls noteable.notes, which AbuseReport does not define.

Why this is code-only

A previous attempt combined this code removal with three foreign key removals and the table drop in one merge request, with the foreign key removals as regular migrations. On production-canary, removing the foreign key to notes could not acquire ACCESS EXCLUSIVE on that table, exhausted its lock retries, and blocked auto-deploy close to the monthly release window (INC-13070). It was reverted: !249546 (merged)

Per avoiding downtime in migrations, dropping a table is a multi-release process: remove application code in one release, then remove the foreign keys and drop the table in separate post-deployment migrations in the next. This merge request is that first release. The schema changes follow:

• Next: remove the notes foreign key, alone, in its own post-deployment migration.
• After that is verified applied on GitLab.com: remove the abuse_reports and organizations foreign keys and drop the table.

What this removes

The mention model and its association, the orphaned AntiAbuse::Reports::IndividualNoteDiscussion, the Mentionable and Noteable wiring on AbuseReport, AbuseReport from Noteable's replyable and resolvable type lists, Note#for_abuse_report? and its four callers, the matching UrlBuilder branch, and the factory and specs.

AbuseReport#project goes too — it existed only so the createNote mutation could call noteable.project. The one other caller, CacheMarkdownField, guards it with respond_to?, so returning nil and not responding at all are equivalent there.

What is deliberately left in place

The table still exists until the follow-up merge requests, so db/docs/abuse_report_user_mentions.yml and its config/organizations/transfer_support.yml entry both stay. The dictionary's classes: key is emptied because the class it named is gone; the dictionary file itself moves to db/docs/deleted_tables/ when the table is dropped.

References

• Reverted merge request: !249546 (merged)
• Drop abuse_report_user_mentions table (gitlab-com/gl-infra/tenant-scale/cells-infrastructure/team#781 - closed)

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.